With wide application of plastic gas pipelines, plastic ball valve is firstly developed and manufactured in USA and widely applied to plastic gas pipeline systems to meet the needs of forming pipeline systems. People have been gradually realized the superiority of plastic ball valves in respects of performances/price and began to replace metal ball valves with plastic ball valves.
For traditional plastic valves, a small-diameter ball valve is integrated by hot melt welding, and a large-diameter ball valve adopts electric fusion welding. The hot melt welding has simple process, low cost and relative low accuracy; and the electric fusion welding has complex process and high cost.
Presently, the valve structure with hot melt welding commonly adopts an integral side-entry structure, in which a ball body, a valve stem and a sealing ring of one side are assembled in a valve body form one end of the valve body, the diameter of the end is equal to or larger than the diameter of a valve ball chamber; the other sealing ring is mounted on a connecting pipe seat, the valve body and the connecting pipe seat are welded together to form an integral ball valve by hot melt welding or electric fusion welding between the inner cylindrical surface with larger diameter where the valve body assembled in the ball body and the outer cylindrical surface of the connecting pipe seat. The valve stem is buckled with a valve stem cap through the buckling part of the valve stem, and the valve stem cap is buckled with the valve body through the buckling part of the valve stem cap. The two sealing rings are respectively fixed on the valve body and the connecting pipe. In assembly, the valve stem cap is firstly buckled with the valve body through the buckling part, then a sealing ring and a supporting ring thereof are mounted in a sealing ring groove and a supporting ring groove from the end with larger diameter of the valve body, next, the valve stem is mounted from the end with larger diameter of the valve body in a way that the buckling part of the valve stem is buckled in the buckling part of the valve stem cap, and finally, the valve ball is mounted; the sealing rings are mounted on a sealing ring mounting ring of the valve body and a connecting pipe seat sealing ring, then the inner cylindrical surface of the insertion-accepting part of the valve body and the outer cylindrical surface of the connecting pipe are heated and molten on a hot melting machine, and when the two cylindrical surfaces are molten to a certain thickness, the connecting pipe is inserted into the insertion-accepting part of the valve body to a certain depth and gradually cooled by keeping pressure.
Such structure has the following defects:
I. The bottom of the valve stem has an end surface on which an O-shaped sealing ring is mounted through an annular groove provided on the end surface, a plane perpendicular to the valve stem is milled on the valve body correspondingly, the buckling part of the valve stem and the buckling part of the valve stem cap are required to cause the valve stem to maintain a proper force pointing at the outside of the valve body, such that the O-shaped sealing ring on the end surface of the valve stem can seal the end surface of the valve stem by a proper pressing force. However, in such structure, the designed pressing force is obtained through the two buckling parts on three parts, which brings more difficulties for manufacture and assembly;
II. The insertion depth of the connecting pipe in the valve body directly affects the axial distance between two sealing surfaces and influences a pre-stressed force. Large insertion depth causes large pre-stressed force and large torques for opening and closing the valve and serious abrasion; small insertion depth brings small pre-stressed force of the sealing surface, thereby causing a hidden danger of inner leakage. In addition, it is difficult to ensure coincidence of central lines of the valve body and the connecting pipe during insertion, the two central lines often do not coincide and form a certain angle, resulting in uneven pressing force between the sealing rings and the ball body on a whole sealing area;
III. From the view of manufacture, because the two sealing rings are respectively positioned on the valve body and the connecting pipe, and process positioning surfaces for machining mounting rings of the sealing rings are respectively positioned on two mounting rings, the axial distance between the mounting rings of the sealing rings after assembly has an error due to the manufacture of the positioning surfaces.